Development of High Fatigue Strength Valve Spring Using Control of White Layer by Nitriding

For the requirements of high power output, lightweight and improved fuel consumption of motorcycles to respond to global environmental needs, high fatigue strength to cope with high stress is becoming more important than ever for valve springs. To satisfy such needs, a new alloy steel that softens less in tempering ^(1),(2) and nitriding that increases surface hardness of valve springs has been developed using oil-tempered silicone-chrome steel wire as the base material. Also, with regards to shot-peening to create compression residual stress on the surface, studies are being performed for multi-stage and high-hardness peening. The research reported in this paper is aimed at an increase of internal hardness after nitriding while maintaining the cold-coiling-ability by adding elements that reduce softening from tempering while reducing non-dissolved carbide in the material. With respect to nitriding, which is a surface treatment of valve springs, conventionally, the hardness is increased without taking the white layer into considerations. Regarding the wire strength, the hardness of the nitrogen-diffused layer and the internal hardness increased to the ultimate limit, and it is already known that the white layer strength is the key factor of fracture, and limits the fatigue strength. Considering that the white layer reduces fatigue strength, the new nitriding method that attains both of the two contradicting requirements; elimination of white layer and increase of hardness of nitrogen-diffused layer, has been developed and reported here.